Nowadays, radiation-sensitive compositions usable particularly for high-performance printing plate precursors must fulfill high requirements.
The discussion of improving the properties of radiation-sensitive compositions and thus also of the corresponding printing plate precursors essentially deals with two different ways. One of them deals with the improvement of the properties of the radiation-sensitive components in the compositions (frequently negative diazo resins or photoinitiators), the other one with the search for novel polymeric compounds (“binders”) which are to control the physical properties of the radiation-sensitive layers. The first way is of particular importance if the sensitivity of printing plate precursors is to be adjusted to certain ranges of electromagnetic radiation. Also, the shelf-life and radiation-sensitivity of the materials are strongly influenced by the nature of such initiator systems.
The latest developments in the field of printing plate precursors deal with radiation-sensitive compositions which can be imagewise exposed by means of lasers or laser diodes. This type of exposure does not require films as inter-mediate information carriers since lasers can be controlled by computers.
High-performance lasers or laser diodes which are used in commercially available image-setters emit light in the wave-length ranges of between 800 to 850 nm and between 1060 and 1120 nm, respectively. Therefore, printing plate precursors, or initiator systems contained therein, which are to be imagewise exposed by means of such image-setters have to be sensitive in the near IR range. Such printing plate precursors can then basically be handled under daylight conditions which significantly facilitates their production and processing. There are two different possibilities of producing radiation-sensitive compositions for such printing plates.
For negative printing plates, radiation-sensitive compositions are used wherein after an imagewise exposure the exposed areas are cured. In the developing step only the unexposed areas are removed from the substrate. For positive printing plates, radiation-sensitive compositions are used whose exposed areas dissolve faster in a given developing agent than the non-exposed areas. This process is referred to as photosolubilization.
However, with regard to the radiation-sensitive compositions in positive systems, there is a certain dilemma since for a high number of copies crosslinked polymers are needed. However, such products are insoluble in the solvents or solvent mixtures suitable for the plate coating so that again non-crosslinked or only slightly crosslinked starting products are needed. The necessary crosslinking can then be achieved by preheating steps which can be carried out at various stages of the plate processing.
A positive system is described in EP-A-0 819 980 where it is assumed that the non-image areas are formed by a reaction of the formed acid with carbon black. The image areas are only formed during a preheating step; for a high number of copies the image areas have to be baked.
Another positive system is described in U.S. Pat. No. 5,658,708. The necessary crosslinking of the layer is already carried out during the drying step of the coating. However, for that purpose the system has to be treated at 120° C. for 10 minutes whereby chemical processes take place which eventually lead to crosslinking. However, the required relatively long heating periods at such high temperatures represent an unacceptable waste of time in today's typical fully automated plate production lines. Baking does not lead to an increase in the number of copies since the crosslinking is partly undone.
Documents EP-A-0 823 327 and WO97/39894 also describe positive compositions. As is the case with many positive systems, they entail the disadvantage that a complicated conditioning step is necessary in order to ensure a sufficient shelf-life of the plates. Furthermore, a baking step is required to obtain high numbers of copies and a good solvent resistance.
Plates which can be imagewise exposed with IR lasers are furthermore known from EP-A-0 672 544, EP-A-0 672 954 as well as U.S. Pat. No. 5,491,046 and EP-A-0 819 985. These plates are negative-working and after imagewise exposure they require a preheating step within a very narrow temperature range which only causes a partial crosslinking of the image layer. In order to meet the highest requirements regarding the number of copies and to show sufficient resistance to printing chamber chemicals an additional heating step—what is referred to as baking—is carried out during which these layers are crosslinked further.
All the systems described so far have the additional disadvantage that a relatively high laser performance (≧150 mJ/cm2) is required; for some applications (e.g. newsprinting) this represents a disadvantage since the provision of the necessary number of exposed printing plates within a short period of time is problematic.
U.S. Pat. No. 4,997,745 describes photosensitive compositions comprising a dye absorbing in the visible range and a trihalomethyl-s-triazine compound. However, these compositions are not sensitive in the IR-range and meet neither today's high requirement regarding photosensitivity nor that of a long shelf-life. In U.S. Pat. No. 5,496,903 and DE-A-196 48 313 photosensitive compositions are described which in addition to a dye absorbing in the IR range comprise borate co-initiators; also, halogenated s-triazines are described as further co-initiators. Although these compositions show an improved photosensitivity, the thus produced printing plates do not meet the present-day requirement of a long shelf-life. After only one month of storage at room temperature, the entire layer of the printing plate has cured to such a degree that an image can no longer be created after exposure and developing of the plate.
Further photopolymerizable compositions with initiator systems are described in U.S. Pat. No 5,756,258, U.S. Pat. No. 5,545,676, JP-A-11-038633, JP-A-09-034110, U.S. Pat. No. 5,763,134 and EP-B-0 522 175.
Radiation-sensitive compositions which show both a high degree of radiation sensitivity and a sufficiently long shelf-life when used in the manufacture of printing plate precursors are presently only known in connection with UV-absorbing dyes (EP-A-0 730 201). However, printing plate precursors using such compositions have to be manufactured and processed under darkroom conditions and cannot be imagewise exposed by means of the above-mentioned lasers or laser diodes. Particularly the fact that they cannot be processed in daylight limits their possibilities of application.
WO 00/48836 describes IR-sensitive compositions comprising besides a polymeric binder and a free radically polymerizable system an initiator system comprising (a) at least one compound capable of absorbing IR radiation, (b) at least one compound capable of producing radicals and (c) at least one polycarboxylic acid comprising an aromatic moiety substituted with a heteroatom selected from N, O and S and at least two carboxyl groups, wherein at least one of the carboxyl groups is bonded to the heteroatom via a methylene group. The compositions may furthermore contain a colorant for increasing the contrast of the image compared to the background after development.
It would, however, be desirable to further improve the IR sensitivity.